You are sending a new comment using the Email to Case Premium New Comment page and observe the following error: Sending e-mail with new comment failed: SendEmail failed. First exception on row 0; first error: ENTITY_IS_DELETED, entity is deleted.

This error indicates that the Contact defined under the Contact Substitution setting was deleted.

To locate the Contact selected for Contact Substitution, please visit Initial Setup:Page 1 of the Email to Case Premium setup wizard and expand the section for Contact Substitution. The Contact will still be listed in the setup wizard even after deletion.

From there, please Undelete the defined Contact from your Recycle Bin or create a new Contact to be used for Contact Substitution.